1. Field of the Invention
This invention relates to a luminescent device (for example an autoluminescent flat display and particularly an organic electroluminescent device or display using an organic thin film as an electroluminescent layer) and a driving method thereof.
2. Prior Art
An organic electroluminescent device (hereinafter also referred to as an organic EL device) is of 1 .mu.m or less in film thickness and can convert electrical energy into light and form a luminescing surface when a current is passed through it and therefore has ideal characteristics as an autoluminescent display device, and in recent years vigorous research and development of these devices has been being carried out.
FIG. 1 shows an organic EL device 10 as an example of a conventional luminescent device. This organic EL device 10 is made by sequentially forming an ITO (Indium Tin Oxide) transparent electrode 5, a hole transfer layer 4, a luminescent layer 3, an electron transfer layer 2 and a cathode (for example an aluminum electrode) 1 on a transparent substrate (for example a glass substrate) 6 by for example vacuum vapor deposition.
When a d.c. voltage 7 is selectively impressed across the cathode 1 and the transparent electrode 5, which is an anode, holes supplied through the transparent electrode 5 pass through the hole transfer layer 4, electrons supplied through the cathode 1 pass through the electron transfer layer 2, the holes and the electrons arrive at the luminescent layer 3 and electron-hole recombination takes place and luminescing 8 of a predetermined wavelength occurs in this luminescent layer 3 and can be observed from the transparent substrate 6 side.
The luminescent layer 3 can be made to contain for example a zinc complex, and may be a layer essentially consisting of zinc complex only (a plurality of different zinc complexes can be used together) or may be a layer comprising a fluorescent substance added to a zinc complex. Also, zinc complex and other luminescent substances such as anthracene, naphthalene, phenanthrene, pyrene, chrysene, perylene, butadiene, coumarin, acridine and stilbene can be used together. This kind of zinc complex or mixture of zinc complex and fluorescent substance can be included in the electron transfer layer 2.
FIG. 2 shows another conventional example, an organic EL device 20 wherein the luminescent layer 3 is dispensed with, zinc complex or a mixture of zinc complex and fluorescent substance is included in the electron transfer layer 2 and luminescing 18 of a predetermined wavelength occurs at the interface of the electron transfer layer 2 and the hole transfer layer 4.
FIG. 3 shows a specific example of a case wherein the organic EL device described above is used as a passive matrix (or simple matrix) display. That is, stacks of organic layers (hole transfer layers 4, luminescent layers 3 and electron transfer layers 2) are disposed between cathodes 1 and anodes 5, these electrodes are disposed in the form of stripes intersecting with each other in the form of a matrix, signal voltages are impressed in time series by a brightness signal circuit 30 and a control circuit 31 comprising a shift register and areas where the electrodes intersect are thereby selectively made to luminesce as pixels. Accordingly, by means of this kind of construction, an organic EL device can be used not only of course as a display but also as an image reproducing device. Also, the above-mentioned pattern of stripes can be provided for each of the colors red (R), green (G) and blue (B) to make a full-color or multicolor display.
It is known that the luminescing brightness of an organic EL device, in the practical brightness area, is roughly proportional to the current (hereinafter also referred to as the device current or the pixel current) flowing through the device (specifically, the pixel).
However, in a passive matrix, when brightness data is supplied to columns as voltages, even if the current-voltage characteristics of the devices are fixed, depending on how many columns pixels of which are to be illuminated in a line and at what brightness, the current flowing through the line changes, and the further a device is along the line electrode (for example one of the above-mentioned electrodes 5) from an electrode connecting to outside, the more greatly the potential of the line electrode side is liable to fluctuate.
Consequently, because the voltage across each pixel is not just the voltage applied to the column electrode (for example, one of the above-mentioned electrodes 1), and fluctuates, there has been the problem that it is not possible to control brightness and it is difficult to display an image. Furthermore, there is a tendency for the devices to increase in resistance with age deterioration, and this makes controlling the brightnesses of pixels by means of voltage even more difficult.
The difficulty of controlling the brightnesses of pixels by means of voltage will now be explained specifically with reference to FIG. 4.
FIG. 4 is an equivalent circuit of a line of a passive matrix. Pixels PX can be regarded as light emitting diodes D connected in a forward direction. The number of columns is n, the resistance of each pixel in the forward direction is R' the resistance of the line electrode 5 between pixels is R' and the resistance of the lead part of the line electrode 5 is R".
Now, considering a case wherein all the pixels are to be illuminated at a certain fixed brightness, the current flowing through each device (each pixel) at this time will be written i. At this time, due to voltage drop the potential of the line electrode 5 at the device PX.sub.1 nearest to a power supply connected to one end (the upstream end as seen from the flow of current) of the line electrode 5 falls by the amount niR" from the power supply voltage, i.e. becomes -niR". The potential of the line electrode 5 at the device PX.sub.n furthest from the power supply falls due to voltage drop by the amount {niR"+(n-1)iR'+(n-2)iR'+ . . . iR'} from the power supply voltage, i.e. becomes {-niR"-(n.sup.2 -n)iR'/2}. On the other hand, when just the furthest device PX.sub.n is to be illuminated at that brightness, the potential of the line electrode at that device falls due to voltage drop by the amount {iR"+(n-1)iR'} from the power supply voltage, i.e. becomes -{iR"+(n-1)iR'}.
Summarizing this yields the following:
1! When all the pixels are to be illuminated at a certain fixed brightness:
The potential of the line electrode at the nearest device PX.sub.1 to the power supply is -niR".
The potential of the line electrode at the furthest device PX.sub.n from the power supply is -niR"-(n.sup.2 -n)iR'/2.
2! When just the furthest device PX.sub.n from the power supply is to be illuminated at a certain fixed brightness:
The potential of the line electrode at the device PX.sub.n is -iR"-(n-1)iR'.
When illuminating a simple matrix of this kind, because the lines are illuminated one by one, each pixel is not continuously illuminated but rather is illuminated for a period of 1/m (where m=the number of lines), and to obtain a brightness of 100 cd/m.sup.2 is it necessary to illuminate the pixels at a peak brightness of 100 m.multidot.cd/m.sup.2.
If m is assumed to be 500 and the current density at this time during luminescing is 100 mA/cm.sup.2 and the pixel size is 0.3.times.0.3 mm in a general EL device, the current is 900 .mu.A. Also, the resistance R' of the line electrode between devices is about 20 .OMEGA. in the case of ITO and about 0.2 .OMEGA. in the case of an interconnection made of a metal such as aluminum. Supposing that the lead length is 5 mm, R" is about 300 .OMEGA. in the case of an ITO electrode and about 3 .OMEGA. in the case of a metal electrode. Also, the number of columns n will be assumed to be 1000.
Here, substituting specific numerical values into the above equations to compare the potentials of the line electrode at the furthest device PX.sub.n from the power supply in the above-mentioned cases 1! and 2! yields the following:
(a) When the line electrode consists of a metal interconnection: PA1 (b) When the line electrode consists of ITO:
1! When all the pixels are to be illuminated at a certain fixed brightness: ##EQU1## 2! When just the pixel PX.sub.n is to be illuminated at a certain fixed brightness: ##EQU2## PA2 1! When all the pixels are to be illuminated at a certain fixed brightness: ##EQU3## 2! When just the pixel PX.sub.n is to be illuminated at a certain fixed brightness: ##EQU4##
Therefore, the potential of the line electrode at the pixel PX.sub.n fluctuates by as much as 92.43 V depending on the display state of the screen.
Therefore, the potential of the line electrode at the pixel PX.sub.n fluctuates by as much as 9243 V depending on the display state of the screen. In this case, it is impossible to make a practical circuit.
From the above results it can be seen that even using metal line electrodes having very low resistance, voltage fluctuations of a level close to 90 V occur, and when ITO line electrodes are used, because the voltage fluctuations become much larger, it is extremely difficult to control brightness by means of voltages applied to the pixels. Indeed, in the case of ITO line electrodes, the voltage fluctuations are so great that it is not even possible to construct a practical circuit.